In a typical communications network a wireless device, communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs). The communications network may also be referred to as e.g. a wireless communications network, a wireless communications system, a communications network, a communications system, a network or a system.
The wireless device may be a device by which a subscriber may access services offered by an operator's network and services outside the operator's network to which the operator's RAN and CN provide access, e.g. access to the Internet. The wireless device may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. user equipment, mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, Machine to Machine (M2M) device or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The wireless device may be portable, pocket storable, hand held, computer comprised, or vehicle mounted devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
The wireless devices are enabled to communicate wirelessly within the communications network. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between the wireless device and a server via the RAN and possibly one or more CNs and possibly the Internet.
The communications network covers a geographical area which may be divided into cell areas. Each cell area is served by at least one base station. The base station may be called Radio Base Station (RBS), evolved Node B (eNB), eNodeB, NodeB, B node, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base station communicates with the wireless device(s) within range of the base station. Each cell is identified by an identity within the local radio area, which may be broadcast in the cell. Thus, the base station in a cell may also be identified using the cell identifier.
Cellular operators have started to offer mobile broadband based on Wideband Code Division Multiple Access/High Speed Packet Access (WCDMA/HSPA) during the last few years. Further, fuelled by new devices designed for data applications, the end user performance requirements are steadily increasing. The large uptake of mobile broadband has resulted in that the traffic volumes that need to be handled by the HSPA networks have grown significantly. Therefore, techniques that allow cellular operators to manage their spectrum resources more efficiently are of large importance.
WCDMA, mentioned above, is an air interface standard found in Third Generation (3G) communications networks and is a commonly used member of the Universal Mobile Telecommunications System (UMTS) family. WCDMA is sometimes used as a synonym for UMTS. HSPA may be described as a combination of High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) and is an evolution of UMTS. HSDPA, also be referred to as 3,5G, turbo-3G or super 3G, is a communications protocol for data transmission in the HSPA family. HSDPA allows networks based on UMTS to have higher data transfer speeds and capacity.
Techniques whereby it is possible to improve the DownLink (DL) performance may involve introducing support for four branch Multiple Input Multiple Output (MIMO), multiflow communication, multi carrier deployment etc. Four branch MIMO may also be referred to as four way MIMO, 4×4 MIMO etc. Since improvements in spectral efficiency per link are approaching theoretical limits, the next generation technology is about to improve the spectral efficiency per unit area. In other words, the additional features for HSDPA need to provide a high area capacity as well as increased user performance; both for a typical user and for cell-edge users. Currently, Third Generation Partnership Project (3GPP) has been working on this aspect of using Heterogeneous networks.
MIMO, mentioned above, is a technology where multiple antennas are used at both the transmitter and receiver to improve communication performance. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. For example, compared to a traditional 1×1 antenna system, a 2×2 MIMO system is expected to deliver significant cell throughput gain. MIMO may also be referred to as smart or intelligent antenna. MIMO may be sub-divided into three main categories, precoding, spatial multiplexing and diversity coding.
Homogeneous Networks:
A homogeneous network is a network of base stations in a planned, regular layout and a collection of wireless devices in which all base stations have similar transmit power levels, antenna patterns, receiver noise floors, and similar backhaul connectivity to the data network. Moreover, all base stations serve roughly the same number of wireless devices. Current communications networks comes under this category for example Global System for Mobil Communications (GSM), WCDMA, HSPA, Long Term Evolution (LTE), Worldwide interoperability for Microwave access (WiMax), etc.
Heterogeneous Networks:
In heterogeneous networks, in addition to the planned or regular placement of macro base stations 101, several pico/femto/relay base stations 103 are deployed as shown in FIG. 1. The macro base station 101 may also be referred to as a macro node or a macro network node. The macro node 101 serves a macro cell 105 and each pico/femto/relay base stations 103 serve a respective small cell 108. Note that the power transmitted by these pico/femto/relay base stations 103 is relatively small compared to that of macro base stations 101. For example 2 Watt (W) as compared to 40 W for macro base stations 101. Therefore, the pico/femto/relay base stations 103 may be referred to as Low Power network Nodes (LPN). These low power network nodes may be deployed to eliminate coverage holes in the homogeneous networks (using macro base stations 101 only) or to improve performance at high loads and enhance capacity, e.g., in traffic hot-spots. Due to their lower transmit power and smaller physical size, the low power network nodes may offer flexible site acquisitions. In FIG. 1 the heterogeneous networks is exemplified to comprise seven (7) pico/femto/relay base stations 103 which may be low power network nodes. However, the skilled person will understand that a heterogeneous network may comprise any number of pico/femto/relay base stations 103 different from 7.
The pico/femto/relay base stations 103 in a heterogeneous network may have:                a) Different cell Identifier (ID) as that of macro cell (different cells)        b) Same cell ID as that of macro cell (soft, shared, or combined cell)Combined Cell in a Heterogeneous Network        
As mentioned above, heterogeneous networks may be divided into two categories, in which:                a) Low power network nodes have different cell IDs as that of the macro network node.        b) Low power network nodes have same cell ID as that of the macro network node.        
FIG. 2 shows an embodiment of a heterogeneous network where low power network nodes create different cells. In FIG. 2, the heterogeneous network comprises a macro network node 101, a first low power network node 103b and a second low power node 103c. The heterogeneous network comprises three cells: cell A 105, cell B 108b and cell C 108c. The cell A 105 is a macro cell is served by the macro network node 101, cell B 108b is served by the first low power network node 103b and cell C 108c is served by the second low power network node 103c. Simulations show that using low power network nodes 103b, 103c in a macro cell 105 offloads the macro network node 101, and the area splitting provided by the low power network nodes 103b, 103c further results in large gains in terms of system throughout as well as cell edge user throughput. A CPICH is a downlink channel broadcast by a network node, such as e.g. the macro network node 101.
One disadvantage of the current technology may be that each low power network node cell creates a different cell ID. Hence, a wireless device needs to do soft handover when moving from one low power network node to a macro network node or to another low power network node. Hence, the number of handovers, as well as the higher layer signaling needed to perform handovers, increases.
FIG. 3 shows the heterogeneous network where the first low power network node 103b and the second low power network node 103c, in addition to the macro network node 101, are part of the macro cell A 105. This is sometimes called a soft cell, shared cell or combined cell. This set up avoids the frequent soft handovers, hence higher layer signaling. A CPICH is a downlink channel broadcast by a network node, such as e.g. the macro network node 101.
FIG. 4 shows an embodiment of a configuration of a combined cell deployment where a central controller 401 in the combined cell 105 takes responsibility for collecting operational statistics information of network environment measurements. The decision of which network node(s) that may transmit to a specific wireless device is made by the central controller 401 based on the information provided by the wireless device or on its own. The cooperation among various network nodes, such as e.g. a low power network node 103, may be instructed by the central controller 401 and implemented in a centralized way.
In a communications network with a combined cell deployment, transmitting the same signal from each network node causes wastage of resources and does not provide capacity benefits when the load of the combined cell is high. One method to increase the capacity of the combined cell deployment may be to reuse the resources (e.g. spreading codes or channelization codes) among various nodes. This is sometimes called spatial reuse. FIG. 5 shows the configuration of spatial reuse between two network nodes, i.e. network node A 501a and network node B 501b in a combined cell. Network node A 501a communicates with a wireless device A 503a and network node B 501b communicates with a wireless device B 503b. Note that these two network nodes 501a, 501b share the same scrambling codes, and also spreading codes or channelization codes. In other words, the network node A 501a and the network node B 501b each comprises the scrambling (S) code S1 and the channelization (c) code c1-14. Thus, S1 and c1 are reused by both network nodes A and B 501a, 501b. 
According to the 3GPP, “Spreading is applied to the physical channels. It consists of two operations. The first is the channelization operation, which transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal. The number of chips per data symbol is called the Spreading Factor (SF). The second operation is the scrambling operation, where a scrambling code is applied to the spread signal.” A channelization code is used to spread the data symbol before they are scrambled. The channelization code may also be referred to as a spreading code. The scrambling code may be divided into 512 sets each of a primary scrambling code and 15 secondary scrambling codes. There is a one-to-one mapping between each primary scrambling code and 15 secondary scrambling codes in a set such that the i:th primary scrambling code corresponds to the i:th set of secondary scrambling codes, where i=0 . . . 511. The set of primary scrambling codes is further divided into 64 scrambling code groups, each consisting of 8 primary scrambling codes. Each cell is allocated one and only one primary scrambling code.
CPICH, mentioned above, may be described as a downlink channel broadcast by a network node with constant power and of a known bit sequence or as a fixed rate downlink physical channel that carries a pre-defined bit sequence. The rate may be 30 kbps and the spreading factor may be SF=256. Its power may be between 5% and 15% of the total network node transmit power. There may be types of CPICH: a Primary-CPICH (P-CPICH) and a Secondary-CPICH (S-CPICH). Some of the characteristics of the P-CPICH are that the same channelization code is always used for the P-CPICH, that the P-CPICH is scrambled by the primary scrambling code, that there is one and only one P-CPICH per cell and that the P-CPICH is broadcast over the entire cell. Some characteristics of the S-CPICH is that an arbitrary channelization code of SF=256 is used for the S-CPICH, that the S-CPICH is scrambled by either the primary or a secondary scrambling code and that there may be zero, one, or several S-CPICH per cell, that an S-CPICH may be transmitted over the entire cell or only over a part of the cell. Another characteristic of the S-CPICH is that an S-CPICH that is intended to be used as phase reference for the second, third or fourth transmit antenna by wireless devices configured in MIMO mode or in MIMO mode with four transmit antennas may be transmitted over the entire cell using the primary scrambling code and the antenna 1 pattern.
A Pilot CHannel (PICH) is a term used in the WCDMA/HSPA RAN1 specification and is a term for a pilot (signal) or reference signal. Examples among the existing PICH include the P-CPICH, which is nothing but a pilot signal (reference signal) that the wireless devices use for certain measurement purposes. Similarly, the S-CPICH is also used by the wireless devices for certain measurements.
It is well known that multiple antennas employed at the network node and the wireless device may significantly increase the system capacity. In a heterogeneous network deployment, by deploying multiple antennas at all network nodes or at a subset of network nodes it may be possible to increase the system capacity by reusing the resources (i.e. sources such as spreading/scrambling codes and frequency) at antennas level as well as at network node level. Unfortunately there are some challenges associated when deploying multiple antennas in a combined cell. One problem is how to identify which network node that is suitable for transmitting data to a specific wireless device when the wireless device is operating in MIMO mode.